Does it taste/smell similar to the original dish? If yes, dig in. If not, add hot sauce prior to digging in.
This is decent advice! I practice it, with a slight modification.
I'm a firm believer in combining a bunch of older leftovers and
recooking them. If cooked properly, there is no issue of getting food poisoning. They may have a possible off-taste; hence the hot sauce or spice.
Leftover uncooked items are another area of interest. Say you have some ground beef, or a chicken breast leftover from the original BIG ASS family pack.
First smell.
Nothing? Good to go.
Mild aroma? In the case of chicken - marinate in some lemon juice. Then cook well. For ground meat cook well. For steak/roasty things - as long as not mechanically tenderized...cook as normal. If mechanically tenderized - turn into ground meat and cook accordingly (unless you like well done steak/roast).
Major nauseating aroma/rainbow meat? Cook the shit out of to eradicate bacteria - then slather on a fuck-ton of hot sauce to mask any bad taste.
Veg and fruit - you aren't likely to get food poisoning as is - soup or juice. If they can't be eaten raw (limp/mushy) - cook.
If you don't get sick - you aren't going close enough to the edge. Push your best-before dates out until you feel a bit dodgy...then pull back a day. :-)